This invention relates generally to systems for radioisotope imaging, and more particularly, to a system for improving radioisotope imaging systems by eliminating the effects of Compton scattered gamma rays.
Cameras of the type used in radioisotope imaging are intended to generate an accurate assessment of the distribution of the isotope within an object under investigation. Such imaging is achieved from the outside of the object, in a noninvasive manner. In scintillation camera systems, gamma rays are detected by use of a solid crystal, which typically may be made of sodium iodide or bismuth germanate (BgO). A collimation system and a computation algorithm are employed to establish the correspondence between the location where the gamma ray is detected and the location within the object where the gamma ray originated. Over a period of time, many gamma rays are detected, and the distribution of the isotope within the object can be determined therefrom.
It is well known that gamma rays which are emitted from within an object are subject to interaction with the matter of the object. Such interaction generally takes the form of Compton scattering, which always results in a diminution in the energy of the interacting gamma ray and a change in its direction of travel. Compton scattered gamma rays therefore approach the scintillation camera along a path which differs from that of its origin. If the fact that these gamma rays are Compton scattered is not detected, and therefore they are processed as though they approach the scintillation camera directly from the point of origin, the gamma rays will be assigned incorrectly, and the determined distribution of radioisotope within the object will be incorrect.
One prior art approach to the problem of misassignment of gamma rays involves the detection of the energy content of the gamma rays received. If a gamma ray received at the scintillation camera has lost a relatively large amount of energy, it is presumed that the gamma ray has been subjected to Compton scattering, and is not accepted for further processing. It is a problem with this known approach, however, that the determination of energy content of the gamma rays is not accurately conducted, and therefore a relatively large energy window has to be used so that most of the unscattered gamma rays, or direct gamma rays, will be accepted. Failure to accept for processing a large number of direct gamma rays will result in poor imaging performance. However, the use of a wide energy window results in the acceptance for processing of a significant number of scattered gamma rays, which also results in poor imaging performance.
In one known system for the rejection of scattered gamma rays, the energy window was set symmetrically about the peak of the energy distribution. This peak corresponds generally to the number of gamma rays as a function of their energy. In other systems, the energy window was set asymmetrically about the energy peak. Other known systems obtain energy distributions for different spatial locations on the planar surface of the scintillation camera, and use different energy windows for different spatial locations.
The use of a single energy window does not achieve the desired result of rejecting the gamma rays which have undergone Compton scattering. As the width of the energy window is reduced, or is set asymmetrically toward higher energies, the ratio of scattered gamma rays to direct gamma rays, which are accepted for processing, decreases. However, the total number of direct gamma rays is also reduced. This results in the further problem that statistical fluctuations in the determined radioisotope distribution have greater effect in degrading the imaging performance. Therefore, to avoid unacceptable statistical fluctuations, the energy window must be made sufficiently large, and a significant number of Compton scattered gamma rays are also accepted for processing. The use of energy windows which vary with spatial location reduces the degradation of imaging performance caused by the statistical fluctuations to an extent, but does not eliminate the problem entirely.
A still further known system for rejection of Compton scattered gamma ray utilizes two energy windows. One such window is set symmetrically about the direct peak of the energy distribution, and the other energy window, of equal width, is set adjacent to the symmetrically set energy window, but at lower energies. A fraction of the image of the radioisotope distribution which is reconstructed from the gamma rays accepted within the energy window of reduced energy is subtracted from the image reconstructed from the direct, or unscattered, gamma rays.
It has been learned that the fraction, k, which is used to multiply the image resulting from the lower energy window before subtraction from the upper window is slightly dependent upon the size of the outer, non-radioactive, water-filled object containing a radioactive sphere. The value of the fraction k might also depend on the shape of the outer object, the size and shape of the radioactive object, the distribution of attenuation coefficients and the distribution of background radioactivity within the outer object. Thus, this known technique is difficult to implement.
It is, therefore, an object of this invention to provide a system which improves the imaging performance of radioisotope imaging systems.
It is another object of this invention to provide a system which eliminates Compton scattered gamma rays from inclusion in the processing of a radioisotope distribution image.
It is also an object of this invention to provide a system which rejects gamma rays which have been Compton scattered from final image formation for any distribution of radioisotope.
It is a further object of this invention to provide a system which rejects gamma rays which have been Compton scattered from final image formation using feasible computation techniques.
It is additionally an object of this invention to provide a system which rejects gamma rays which have been Compton scattered from final image formation, and which is suitable for singlephoton projection (planar) imaging.
It is yet a further object of this invention to provide a system which rejects gamma rays which have been Compton scattered from final image formation, and which is suitable for single photon emission computed tomography (SPECT).
It is also another object of this invention to provide a system which rejects gamma rays which have been Compton scattered from final image formation, and which is suitable for positron emission computed tomography (PET).
It is yet an additional object of this invention to provide a system which rejects gamma rays which have been Compton scattered from final image formation, and which is suitable for use with radioisotope imaging systems which use sodium iodide (NaI) scintillation crystal material.
It is still another object of this invention to provide a system which rejects gamma rays which have been Compton scattered from final image formation, and which is suitable for use with radioisotope imaging systems which use bismuth germanate (BgO) scintillation crystal material.
It is a yet further object of this invention to provide a system which facilitates quantitatively accurate reconstruction of a radioisotope distribution image.
It is also a further object of this invention to provide a system which rejects gamma rays which have been Compton scattered from final image formation, but which allows for all of the direct gamma rays which are detected to be used for formation of the image.
It is additionally another object of this invention to provide a system which utilizes in the formation of a radioisotope image additional information obtained from a scatter-free calibration step.
Yet another object of this invention is to eliminate the need for repeated remeasurement of a calibration function, whereby variation in the relative state of the system is tolerated.
A still further object of this invention is to provide a system for eliminating Compton scattered gamma rays which can be applied to any number of tomographically reconstructed planes through the body.
An additional object of this invention is to provide a system for eliminating Compton scattered gamma rays while reducing computation time.